: In the mammalian kidney, isoform 3 of the Na/H exchanger (NHE3; gene symbol: Nhe3) has been identified as one of the major ion transport mechanisms in the apical membrane of the proximal tubule, and accounts for up to 60% of the total reabsorption of salt and fluid in this segment. Consistent with this dominant role in mediating reabsorption in the kidney, derangements in NHE3 have been implicated in several forms of hypertension. However, regulation of the activity of this transporter, especially in response to known physiologic stimuli, is not well understood. Likewise, the potential contribution of this transporter to important renal homeostatic processes has not been explored in the context of whole kidney function. Since the lack of information in this regard is largely due to the absence of an effective pharmacologic inhibitor of Na/H exchange, the recent development of an Nhe3-knockout mouse provides a unique and important opportunity to specifically evaluate the contribution of proximal Na/H exchange to the maintenance of overall Na+ fluid balance. The primary goal of this proposal, then, is to define the role of proximal Na/H exchange in the various mechanisms that are known to regulate Na+ excretion. Comparisons between Nhe3 knockout and wild type mice will allow the applicants to specifically evaluate the contribution of NHE3 to Na+ excretory responses. For these studies, they will use an Nhe3 knockout mouse in which intestinal expression of NHE3 has been restored using transgenic approaches. The specific aims of this proposal are: 1) To examine the natriuretic and diuretic response to extracellular volume expansion (ECVE) in Nhe3 knockouts, in order to test the hypothesis that inhibition on NHE3 activity plays an important role in the natriuretic response to ECVE. 2) To examine pressure-natriuresis responses in Nhe3 knockouts, in order to test the hypothesis that pressure-induced natriuresis in normal animals is in part due to inhibition of NHE3 activity. 3) To examine nitric oxide (NO) signaling pathways in the control of Na+ excretion in Nhe3 knockouts, in order to test the hypothesis that one of the mechanisms by which NO influences Na+ excretion is through a direct inhibitory effect on NHE3. 4) To examine the role of angiotensin II (Ang II) in mediating Na+ excretion in Nhe3 knockouts, in order to test the hypothesis that Ang II influences Na+ excretion by directly regulating NHE3. These experiments, which combine a unique animal model with state-of-the-art micropuncture techniques, will provide new insights regarding the role of proximal tubule Na/H exchange in regulating Na+ excretion, and will lay the groundwork for future studies which will explore the signal transduction pathways involved in these responses.